Objective: To explore the diagnostic value of progesterone in 21-hydroxylase deficiency (21-OHD). Methods: One hundred and ninety-three patients (female: 127, male: 66) with 21-OHD were recruited at Peking Union Medical College Hospital between January and December 2015. At the same time, 253 patients (female: 162, male: 91) with other disease were included as control, and their sex hormone and 17-OHP were measured simultaneously. The correlation between 17-alpha-hydroxyprogesterone (17-OHP) and progesterone was analyzed and the probability of progesterone as the diagnostic biomarker of 21-OHD was determined. Results: There was obvious positive correlation between progesterone and 17-OHP levels in subjects with different age and gender, and the correlation coefficient was from 0.775 to 0.878 (all P<0.001). Receiver operating characteristic curve (ROC) analysis showed that the area under curve was from 0.930 to 1.000 (all P<0.001) when using progesterone as the diagnostic biomarker. Conclusions: Obvious positive correlation exists between progesterone and 17-OHP in patients with 21-OHD. The measurement of progesterone can be an assistant method for the diagnosis of 21-OHD.